Hematopoiesis is the process by which hematopoietic pluripotent stem cells mature into functional blood cells (i.e., red blood cells (erythrocytes), white blood cells (T-cells, B-cells, NK cells, dendritic cells, basophils, polymorphonucleated cells, macrophages, monocytes, and eosinophils), and platelets). In the current model of hematopoiesis, all blood cells begin as pluripotent stem cells. These pluripotent cells are partitioned between resting and proliferating compartments, and during hematopoiesis some of these cells are transformed to committed progenitors of red blood cells, white blood cells, or platelets by the influence of multiple growth factors and cytokines. These committed progenitor cells undergo further differentiation and commitment influenced by growth factors and cytokines. The committed cells are also partitioned between resting and proliferating compartments; however, many more of these cells are proliferating. These committed progenitor cells give rise to morphologically identifiable immature precursor cells (i.e., blasts), which populate the marrow. These precursor cells mature further and eventually enter the blood where they are influenced further by growth factors and cytokines.
High levels of production of mature blood cells are needed to replace their rapid turnover in the body (tens of billions of cells per day in the human with rapid increments during times of physiologic stress). Maintenance of blood cell production requires a highly cytokine responsive progenitor cell pool with prodigious proliferative capacity and a smaller population of stem cells intermittently feeding daughter cells into the proliferative compartment. The proliferative activity of these very important hematopoietic stem cells has been hypothesized to be highly restricted to prevent susceptibility to myelotoxic insult or consumption of the regenerative cell pool (Mauch et al. Bone Marrow Transplant 4:601-607, 1989; Mauch et al. Int. J. Radiat. Oncol. Biol. Phys. 31:1319-39, 1995; Gardner et al. Exp. Hematol. 25:495-501, 1997; each of which is incorporated herein by reference). Once these stem cells embark on a path of high proliferation, they appear to survive only 1 to 3 months (Drize et al. Blood 88:2927-2938, 1996; incorporated herein by reference). Hematopoietic tissue has therefore been thought to be organized such that stem cells are relatively quiescent and cytokine resistant, but that their more differentiated offspring have extremely robust proliferative potential (Ogawa Blood 81:2844-53, 1993; incorporated herein by reference). The dichotomy of resistance to proliferative signals by stem cells and the brisk responsiveness by progenitor cells is a central feature of hematopoiesis, and the molecular mechanisms governing it are not well understood.
Stem cells and progenitor cells are used in research, bone marrow transplantation, and gene therapy; however, stem cell expansion without loss of multipotentiality is a problem. Current technology is based on driving stem cells to proliferate with superphysiologic doses of cytokines. These cytokines unfortunately have pleiotropic effects which include differentiation of primitive cells. The result of these techniques is expanded cell numbers but a loss of multipotentiality. Due to these problems in expanding stem cell populations, one third of patients are currently denied autologous bone marrow transplantation because of inadequate stem cell numbers. For example, cord blood stem cells are the best source of stem cells for minority groups, yet they are inadequate in number to transplant adults.
Stem cell gene therapy has been a failure to date largely due to the inability to achieve gene transfer in quiescent cells. Both bone marrow transplantation and gene therapy would be revolutionized by successful stem cell and progenitor cell expansion technology.